Field of the Invention
The present invention relates to image forming apparatuses such as laser beam printers, digital copiers, and digital facsimiles.
Description of the Related Art
Electrophotographic image forming apparatuses have an optical scanning unit for forming an electrostatic latent image by exposing a photosensitive member to light. The optical scanning unit exposes the image forming surface of the photosensitive member to light by emitting a laser beam based on image data, reflecting the laser beam with a rotating polygonal mirror, causing the laser beam to pass through a scanning lens, and thereby irradiating the photosensitive member with the laser beam. The scanning lens is a lens that has so-called fθ characteristics. fθ characteristics are the optical characteristics that form an image of a laser beam on the surface of a photosensitive member such that the spot of the laser beam on the surface of the photosensitive member moves at a constant speed while a rotating polygonal mirror is rotating at a constant angular velocity. By using a scanning lens with the fθ characteristics, the exposure length (i.e., the dot width) per pixel in the main scanning direction is maintained at a constant length.
Scanning lenses with the fθ characteristics have a relatively large size, and their cost is high. In order to reduce the size and the cost of an image forming apparatus, it is conceivable to not use a scanning lens, or otherwise use a small scanning lens without the fθ characteristics. Japanese Patent Laid-Open No. S58-125064 discloses changing the image clock frequency of an optical scanning unit in which the spot of a laser beam on the surface of a photosensitive member does not move on the surface of the photosensitive member at a constant speed such that the widths of dots that are to be formed on the surface of the photosensitive member are equal.
According to Japanese Patent Laid-Open No. S58-125064, there is the advantage that the widths of dots to be formed on the surface of the photosensitive member are equal despite the use of a scanning lens without the fθ characteristics. However, since the moving speed of the spot of the laser beam on the surface of the photosensitive member is not constant, the exposure amount per unit area differs for dots in an edge portion of the photosensitive member and dots in a central portion of the photosensitive member. In other words, even if the same image data is used, the image density of dots in an edge portion of the photosensitive member and the image density of dots in a central portion of the photosensitive member differ from each other, which causes density unevenness and color unevenness.